[appveyor] Run pyinstaller last

appveyor.yml

@@ -16,8 +16,8 @@ environment:
     # For Python versions available on Appveyor, see
     # https://www.appveyor.com/docs/windows-images-software/ or
     # https://www.appveyor.com/docs/linux-images-software/
-    - pydir: 'C:\Python36'
     - pydir: 'C:\Python37-x64'
+    - pydir: 'C:\Python36'
   global:
     py: '%pydir%\python.exe'
     APPVEYOR_SAVE_CACHE_ON_ERROR: true